<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>计算圆的面积</title>
</head>
<body>
    
</body>
<script>
let r = prompt('请输入圆的半径')
let area = Math.PI *Number(r)  * Number(r)
alert('圆的面积是' + area)
</script>
</html>